Organic

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Ind. Eng. Chem. Res. 2009, 48, 1468–1475

MATERIALS AND INTERFACES Novel Synthesis and Characterization of Yellow Inorganic/Organic Composite Spheres for Electrophoretic Display Jihai Duan, Yaqing Feng,* Guang Yang, Wenliang Xu, Xianggao Li, Ying Liu, and Jing Zhao School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China

Yellow CdS/wax nanocomposite spheres were fabricated in an aqueous solution by an emulsion method, in which the CdS nanoparticles were adsorbed on the surface of a wax core with positive charge by electrostatic self-assembly. A thin shell of SiO2 was then coated to the yellow CdS/wax spheres by the hydrolysis of Na2SiO3 in the same aqueous solution to enhance the optical and mechanical properties and the charge load of the composite spheres. The product was characterized by transmission electron microscopy, scanning electron microscope, atomic force microscopy, thermogravimetric analysis, and X-ray powder diffraction, which showed that the SiO2 walls of spheres were compact and part of CdS crystals dispersed inside, the density of composite spheres being about 1.3 g/cm3, which match most of the suspensions. Fourier transform infrared spectroscopy, and energy dispersive X-ray spectrometry showed the component of composite spheres. Dynamic light scattering showed the diameter distribution of composite spheres was between 100∼400 nm. Zeta-potential measurement proved that the SiO2/CdS/wax spheres had a higher charge load, and Ultraviolet-visible spectra showed that the SiO2/CdS/wax spheres had a better optical property. Therefore, this type of composite spheres had the merits of low density and strong durability in environments. The response behavior of the microencapsulated electronic ink of the composite spheres has been measured. This novel method is expected to produce various inorganic/organic nanocomposite spheres with potential application in the fields of electronic paper and other material science. Introduction In recent years, electronic paper has become a most appealing application due to its low cost, low weight, good flexibility, and low power consumption, as can be seen in the examples of the microcapsule-type and microcup electrophoretic display, the twisting ball display, the cholesetric liquid crystal display, and the electrowetting display.1-5 Among these different materials, the microencapsulated electrophoretic display could be most promising reflective displays, as they can offer the advantage of lower manufacturing cost by roll-to-roll fabrication. Electrophoretic display imaging is based on the movement of colored pigments in a solvent, with either clear or an optically contrasting color under the influence of an electric field. That is, certain charged particles inside a small capsule can be driven to migrate through the solvent toward an electrode of opposite charge. When the voltage of the opposite sign is applied, the observer will see the color change. Therefore, the characteristics of electrophoretic particles will become the key factor influencing the imaging quality. At present, the white-black electronic book has been commercialized, however color E-book or display is studied extensively, resulting in a suitable color particle becoming the most important element in achieving the product. As we know, the realization of color imaging in electrophoretic display is based on the tricolor (yellow, magenta, and cyan) theory of reflex light. The target of this study is the adoption of a relatively easy method of fabricating colored particles with good quality, shape, and light reflection. Generally, many kinds of materials can be used as electrophoretic particles, such as metal oxide, polymeric particles, and pigment.6-8 However, the disadvantages of the particles men* To whom correspondence should be addressed. Tel: 86-2227401824. Fax: 86-22-27401824. E-mail: [email protected].

tioned above are obvious: (a) the dimension and shape of organic pigment or polymer can easily be enlarged or changed in organic suspending solvent; in addition, typical organic pigment cannot be used outdoors since it exhibits major changes in color upon even short-term exposure; (b) the density of inorganic pigment or metal oxide (usually F > 4) is not usually matched to that of the suspending medium (F < 2). In addition, polymer coating for the reduction of the density of particles will cause new flaws, such as deformation of organic particles in organic suspensions, weakness of light reflectance, and of chroma of the inorganic pigment. Although inorganic pigment particles have such high density that it is difficult to disperse them into the suspending medium, and their irregular shape affects the efficiency of reflection, they are still more attractive due to their strong durability in environments such as air, organic solvent, and sunlight. In order to overcome the deficiencies of inorganic pigment in shape and density, composite particles were synthesized by coating the inorganic pigment onto the organic core. Recently, the deposition of nanoparticles onto the core as a coating material was reported.9-12 However, in general, it is very difficult to coat particles with metal oxide because the corresponding precursors are reactive, which causes the core to aggregate or the metal oxide to form separate particles. In addition, inorganic/organic polymer nanocomposite spheres have been prepared. For example, Caruso13 and his colleagues prepared inorganic/polymer microspheres through the colloid template electrostatic layer-by-layer self-assembly of oppositely charged inorganic nanoparticles and polymer multilayers, but this method is labor-intensive. The yellow CdS crystal particle contains very good optical properties as well as electrical and chemical attributes, thus it has been used extensively in many

10.1021/ie800416w CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008

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Figure 1. Schematic illustrating of the formation mechanism of composite electrophoretic sphere.

fields. At present, many methods of synthesizing CdS nanocomposite spheres have been reported, such as the emulsification method,14 the template method15-17 and an ultrasonic method.18 However, the operating conditions of these methods are rigorous and the processes are time-consuming. In this work, we adopted a facile method, which combined the merits of emulsification and electrostatic self-assembly, to coat cationic palm wax cores with yellow particles of CdS. First, the wax cores were prepared by emulsification at a high temperature, then CdS nano crystal particles were precipitated onto the surface of the wax cores during the reaction between Na2S and CdSO4. The diameter of the composite particle can be controlled by changing the quantity of surfactant in the process of wax emulsification. In addition, coating a thin SiO2 shell19 onto the surface further strengthened the optical properties, charge load, and mechanical intensity of composite particles. Experimental Section Materials. Palm wax (a kind of ester imported from Brazil, food grade, melting point 90 °C, F ) 0.97, abbr: wax), 3CdSO4•8H2O, Na2S•9H2O, Na2SiO3•9H2O, cetyltrimethylammonium bromide (CTAB), and ethanol were purchased from Guangfu Chemical Co. Hyperdispersant CH-8 was purchased from Zhenggao (Shanghai) Polymer Company. All other chemicals used in this study were of reagent grade. Preparation of the Organic Core. The organic core was synthesized by the facile emulsification method. A 1-g portion of wax and 0.1 g CTAB were added to 50 mL of pure water and stirred for 0.5 h at 96 °C. Subsequently, a hard core with a positive charge was obtained as the temperature cooled down to 70 °C, and then 100 mL deionized water was added inside and stirred violently for 0.5 h. Fabrication of the Yellow Spheres. As a benchmark to 1 g wax, a 1.2 g/cm3 of density of the CdS/wax composite nanoparticle was designed in order to match most kinds of organic or inorganic liquids used as suspension. Thus 0.57 g 3CdSO4•8H2O and 0.53 g Na2S•9H2O were needed, respectively. The fabrication strategy involved two key steps, as shown in Figure 1. First, the wax spheres were coated with one layer of CdS by an electrostatic attraction technique. A 20-mL portion of Na2S solution (0.53 g Na2S•9H2O inside) and 20 mL CdSO4 solution (0.57 g 3CdSO4•8H2O inside) were dripped into the wax emulsion, respectively, commencing with the Na2S solution then the CdSO4 solution. The time difference between two kinds of drops should be above 5s. This process was finished within 1 h, and then stirred slowly for 5 h at 70 °C. Second, for coating SiO2 to intensify the yellow spheres, 20 mL of 0.5 M Na2SiO3•9H2O solution was dripped, and 2 M H2SO4 solution was dripped also during whole process to adjust the pH to 9∼10. After this process, the temperature was increased to 85 °C and reaction conditions were still maintained for 4 h. The resulting product was collected by centrifuge and washed several times with absolute ethanol, and then vaccuum-dried at 60 °C for 4 h.

Characterization. Fourier transform infrared (FTIR) spectroscopy was carried out on a NICOLET 380 spectrometer. The UV-vis reflectance spectrum was recorded at room temperature on a SHIMADZU UV-365 spectrophotometer. The sample was identified by X-ray power diffraction (XRD) employing a scanning rate of scanning rate of 0.02 deg/s in the 2θ range from 5 to 80°, using a RIGAKU D/MAX 2500V/PC X-ray diffractometer equipped with graphite monochromatized Cu K R radiation (λ ) 0.154056 nm). Thermogravimetric analysis (TGA) was performed with a Pyris Diamond TGA/DTA 6300 instrument under a stream of air, and the sample was heated at 10 °C/min from 36 to 800 °C. The transmission electron microscopy (TEM) was performed on a JEOL-100CX-II with an accelerating voltage of 200 kv. The field-emission transmission electron microscopy (FTEM) image was obtained on a TECNAI G2F-20 field-emission transmission electron microscopy. An AJ-IIIa atomic force microscopy (AFM) and a PHILIPS XL-30ESEM scanning electron microscope (SEM) were utilized to study the surface morphology of nanoparticle. The zeta potential of nanoparticle was measured on a JS94J microelectrophoretic measurement instrument. The component of composite spheres was detected by energy dispersive X-ray spectrometry (EDX) on an Oxford Inca Energy 300. The diameter distribution of CdS/wax composite spheres was measured by Dynamic Light Scattering (DLS) on BI-200SM. Results and Discussion Control of Core Size and Thickness of CdS Shell of Composite Particles. Figure 2 showed the TEM images of a series of CdS/wax composite particles with outside diameters between 50 and 100 nm and with CdS shell thicknesses between 10 and 20 nm, which were synthesized by varying the CTAB/ wax and CdSO4/wax weight ratio while keeping the Na2S/CdSO4 Mole ratio at 1:1 and maintaining all other conditions at constant levels. It can be concluded that the diameter of wax core decreased when increasing the CTAB/wax weight ratio (Figure 2, parts a and b). The CdS shell thickness of composite particles can also be controlled by altering the CdSO4/wax weight ratio, while keeping Na2S/CdSO4 Mole ratio at 1:1. The reason is that S2- congregated around the positive core because of electrostatic attraction after the Na2S was added, and when CdSO4 was added, the CdS sediment coated the core to form a new core with a positive charge. As the quantity of Na2S and CdSO4 increased, the thickness of the shell increased also, due to the electrostatic electronic attraction between the core with a positive charge and S2-. TEM images indicated that at high CdSO4/ wax ratios, giving thick shells, the composite particles were robust and maintained their spherical shapes (Figure 2c). However, at low CdSO4/wax ratios, the resulting CdS shell was thin and flexible. The light beam caused a high temperature on the surface of the spheres so that the wax cores were melted and deformed (Figure 2, parts a and b). In addition, the diameter distribution of composite spheres was measured by DLS. Figure 3 shows the diameter distribution of CdS/wax composite spheres prepared under a given condition as described in Figure 2a. It can be concluded that composite spheres were not monodispersed.

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Figure 3. CdS composite spheres size distribution (CTAB/wax is 0.15:1, 3CdSO4•8H2O/wax is 0.15:1).

Figure 4. FT-IR spectra of wax sphere, CdS-wax sphere and SiO2/CdS/ wax spheres.

Figure 2. TEM images of CdS/wax composite spheres. (a) CTAB/wax is 0.15:1, 3CdSO4•8H2O/wax is 0.15:1, (b) CTAB/wax is 0.2:1, 3CdSO4•8H2O/ wax is 0.3:1, (c) CTAB/wax is 0.24:1, 3CdSO4•8H2O/wax is 0.6:1 (weight ratio).

Characterization of Composite Particles. Figure 4 compared the FT-IR spectrum of SiO2 (coat)-CdS (middle)-wax (core) spheres with that of CdS (coat)-wax (core) spheres and of wax spheres, respectively. Characteristic peaks of the waxcore are shown in the spectrum: C-H stretching at about 2920 cm-1 and 2829 cm-1, CdO stretching at 1750 cm-1, CH2 bending at 1454 cm-1. Because no absorption for CdS exists within the scope of the IR spectrum, the curve of the CdS-wax spectrum (in the middle) is the same as that of the wax-core. However, TEM images (shown in Figure 2) proved that the CdS coated on the surface of wax core. The SiO2 structure was confirmed in the lowest spectrum: Si-O stretching at 1100

Figure 5. Energy dispersive X-ray spectrometry (EDX) of SiO2/CdS/wax composite spheres.

cm-1, Si-O-Si bending at 477 cm-1and Si-OH stretching at 952 cm-1. Si-O stretching spectrum and the followed TEM image (Figure 9b) proved that SiO2 coated the surface of CdS/ wax composite spheres. In addition, the EDX data (shown in Figure 5) also proved the components of SiO2/CdS/wax composite spheres. From the EDX data, it can be seen that the component of composite spheres included Si, C, O, S, and Cd. Combined with the XRD data, it can be speculated that the composite consisted of SiO2 and CdS and organic material. Figure 6 showed the surface electric charge characteristics of the wax-core, the CdS-coated core, and the SiO2/CdS/wax composite spheres. Before CdS coating, the wax-cores were with positive charges over a wide range of pH values because the CTAB was absorbed on the surface of the cores during the emulsification process. However, the surface charge of the CdS-


Figure 6. Zeta potential of wax-core, CdS-wax, and SiO2/CdS/wax spheres.

Figure 9. (a) XRD patterns of CdS/wax and SiO2/CdS/wax composite spheres; (b) FTEM of CdS crystal nanoparticle (black dot) dispersed into SiO2 shell, black bar is 10 nm.

Figure 7. Effect of CH-8 in tetrachloroethylene on electrophoretic mobility of SiO2/CdS/wax composite spheres.

Figure 8. The approximate structure of hyperdispersant CH-8.

coated wax particles changed from positive to negative when the pH > 5.5 (isoelectric point). CdS can be solved when pH < 3, so its Zeta value was not measured at pH ) 2. When SiO2 coated the composite CdS/wax particles, the isoelectric points of SiO2/CdS/wax particles were found to be pH ) 3.52. Obviously, SiO2 shell induced higher value of Zeta potential of yellow electrophoretic spheres. For electronic ink applications, the electrophoretic particles with higher charge are needed so that they will have a proper mobility in the electronic field. In fact, the SiO2/CdS/wax composite spheres were dispersed into tetrachloroethylene in this work to simulate electrophoretic mobility of E-ink. Hyperdispersant CH-8 was added to the tetrachloroethylene suspension in order to maintain the stability of whole system; meanwhile, CH-8 enhanced the electric charge on the surface and the electrophoretic mobility of composite spheres. Figure 7 showed the effect of CH-8 on the electrophoretic mobility of the SiO2/CdS/wax composite spheres. The

electrophoretic mobility value increased as CH-8 concentration increased; however, it showed the mobility value no longer increased when CH-8 content was more than 8 wt%. The reason can be speculated that as the concentration of CH-8 increased, the number of effective charging sites of composite spheres increased because of the interaction between anchor groups of CH-8 and polarity groups on the surface of composite spheres, resulting in higher electrophoretic mobility of the spheres. However, when the absorption of CH-8 on the surface of spheres was saturated, the value of the electrophoretic mobility of spheres would remain constant. Figure 8 shows the approximate structure of hyperdispersant CH-8. When CH-8 was added to the suspension, its anchor group -NH2 would combine with -OH on the surface of composite spheres by hydrogen bonding, and its modified polypropylene chain would enclose the composite spheres to keep the spheres from agglomeration. In this way, the whole system can maintain stability for a long time. The XRD patterns of the core-shell and core-middle-shell spheres were shown in Figure 9a, which were consistent with the structure of composite particles. Analysis of the CdS/wax core-shell particles confirmed that deposited CdS films can be indexed as cubic CdS crystal according to all the broad diffraction peaks (111, 200, 220, 311), and an average crystallite size of about 5nm was roughly estimated by analyzing the line width of the (111) diffraction peak based on the Scherrer formula,20 and the size was in good agreement with the results obtained from TEM image (Figure 9b). Organic wax showed the very high but narrow peaks here. The lower XRD line revealed that the SiO2-coating weakened the intensity of CdS diffraction peaks except the characteristic peak of SiO2 at 22°. The surface morphology was studied by Atomic force microscopy (AFM), Figure 10 illustrats the typical AFM images of the wax-core (Figure 10a), the CdS-coated wax spheres


Figure 11. SEM images of (a) wax-core, (b) CdS coated wax, and (c) SiO2/ CdS/wax composite sphere.

Figure 10. AFM images of (a) wax-core, (b) CdS coated wax, and (c) SiO2/ CdS/wax composite sphere.

(Figure 10b), and the SiO2/CdS/wax composite spheres (Figure 10c), which were prepared under the conditions described in the Experimental Section of this work. AFM measurement revealed that the surface of the wax was very smooth before being coated with CdS. While the CdS-coated wax had a relatively rough surface when covered with porous CdS layer, and the SiO2/CdS/wax spheres had a smooth surface compared with the CdS/wax. In order to further illustrate the surface morphology, SEM images were also taken. Figure 11a showed that the surface of the wax-core was smooth; however, there

were many cracks on the surface of wax-core when it was under the dried measurement condition. Figure 11, parts b and c, shows the surface morphology of CdS/wax and SiO2/wax composite spheres, respectively, and it could be found that the CdS-coated wax had a relatively rough surface and the SiO2/CdS/wax spheres had a smooth surface with the same results as Figure 10, parts b and c. The results were consistent with the TEM images. The corresponding transmission electron microscopy (TEM) images for CdS/wax composite spheres were shown in Figure 12. The strong contrast between the dark edges and pale center proved the composite structure because the light beam can permeate the wax core. The thickness of the shell is about 30 nm, and the surface is rough. Figure 13 showed the TEM images of the SiO2/CdS/wax composite spheres. In Figure 13, parts a and c, it can be seen that the surface of the sphere is smooth. For a further investigation of the inner structure of the SiO2 and CdS shells, images at high magnification were also taken


Figure 12. TEM image of CdS/wax composite sphere (weight ratio: CTAB/ wax is 0.1:1, 3CdSO4•8H2O/wax is 0.57:1).

(Figure 13b). From the image, it can be concluded that the wall of spheres is compact and the SiO2 and CdS layers have no obvious interface, but the whole shell becomes gradually darker from the exterior to the interior. It is very clear that the CdS crystal nanoparticles, which are on the exterior of the CdS layer, dispersed into the SiO2 shell (Figures 13b and 9b). It can be speculated that SiO32- or Si(OH)4 gel penetrated into the porous CdS shell because of the static electronic attraction between positive core and SiO32- before the hydrolysis of Na2SiO3. Then Na2SiO3 commenced to hydrolysis, not only in the CdS porous layer, but also in solution. This process caused that CdS crystals dispersed into SiO2 layer so that there was no obvious interface between the CdS shell and the SiO2 shell. Figure 14 illustrates TGA thermograms of the CdS/wax and SiO2/CdS/wax spheres. With the CdS/wax spheres first, the initial weight loss of 1% (up to 200 °C) was due to the evaporation of physically absorbed water, and the subsequent loss of 2% (200-300 °C) was due to the decomposition of rudimental surfactant (such as CTAB) or combined water. Between 300 and 550 °C, a weight loss of 80% was attributed to the removal of the palm wax, and the residual weight of 15% should be the weight of the CdS. This indicated that the weight fraction of CdS should be calculated as follows: WCdS ) [15/ (80 + 15)] × 100% ) 15.5%. Therefore, the calculated weight fraction of the wax should be 84.5%. As is known, the density of CdS is 4.8 g/cm3 and the density of palm is 0.97 g/cm3; therefore, the density of composite particles (CdS/wax) can be calculated: (4.8 × 0.97)/(0.97 × 0.155 + 4.8 × 0.845) ) 1.13 g/cm3 (approximate designed value, 1.2 g/cm3), which can match many kinds of organic suspensions. Second, it can be concluded from lines b and c that SiO2 formed a compact inorganic layer on the surface of the CdS/ wax composite spheres, resulting in the inflection on lines b and c at 300 °C. Before this temperature, all of the wax enveloped inside the spheres had almost no loss by heat. When T > 300 °C, the SiO2 layer began to crack, and melted or gasified wax was emitted from spheres and rapidly taken out by the airflow in the instrument. This was the reason why the weight loss of b and c samples were so fast. By calculation, the density of the SiO2/CdS/wax spheres was about 1.3 g/cm3 when the weight ratio of WSiO2 and WCdS was 1:1. Meanwhile, we measured the density of CdS/wax and the density of SiO2/ CdS/wax by pycnometer method, and density of CdS/wax is 1.15 g/cm3, SiO2/CdS/wax is 1.28 g/cm3. In fact, after the composite powders were dispersed into tetrachloride (F ) 1.5) by ultrasonic then kept quiescent for 48 h, we did not find the sediment on the bottom with the naked eye. That all of the

Figure 13. Parts (a) and (c) are FTEM images of SiO2/CdS/wax composite spheres. Part (b) is an FTEM image of the wall structure.

particles floated on the top of the suspension proved that the density of the composite spheres was homogeneous. The optical properties of the CdS/wax and SiO2/CdS/wax nanocomposite spheres shown in Figure 15 were determined by UV-vis spectroscopy in a reflectance mode, which is helpful for theoretically researching whether the composite sphere is a highly efficient electrophoretic nanoparticle for color imaging, because an electrophoretic particle used in reflective display should have high reflective efficiency. Figure 15 shows the recorded spectra of powders of two samples. The reflection of CdS/wax spheres was strong in the visible regions of 700∼800 nm (line a); however, it was surprising that their reflection ability was significantly enhanced when the surface of CdS/wax spheres was coated with SiO2 (line b). Even though reflectance of CdS/wax within 570∼700 nm dropped down obviously, SiO2/ CdS/wax still kept a steady reflectance from 570 to 700 nm.


Figure 14. TGA analysis of CdS/ wax and SiO2/CdS/wax nanospheres.

Figure 15. UV-vis reflectance spectra of (a) SiO2/CdS/wax spheres, (b) CdS/wax sphere. All of the samples were dried thoroughly in a vacuum desiccator at 60 °C for 48 h.

The differences between the reflection spectra of the CdS/wax and SiO2/CdS/wax spheres most likely arose from the good crystalline quality of SiO2 nanoparticle in the composite systems.21 Moreover, by comparing curves a and b, it can be found that the shape of the two curves is similar, that is, their reflectance has a sharp attenuation when the light wavelength is less than 600 nm, although there is an obvious difference in reflectant strength between them, that is, sharper attenuation for the SiO2/CdS/wax spheres. Consequently, it can be concluded that the characteristic reflection in the UV-vis spectra of composite structures can be attributed to the synergistic effect of SiO2 and CdS, which should cause the composite particles to be more sensitive to the special wavelength of visible light. Since the SiO2 formed a compact, smooth, and transparent shell, when the CdS nanoparticle and SiO2-coatings were in contact, an interfacial reflection process was promoted in a favorable way, resulting in the enhancement of reflective strength and the tiny blue shift of the absorption edge.22-24 Therefore, this kind of composite sphere is particularly interesting when used for electrophoretic display because its high reflectance should enhance the imaging resolution of the display. Simulation of Response Behavior of Composite Particles in Electrophoretic Display. The microcapsules were fabricated according to our work.25 The response behavior of the microencapsulated electronic ink was first investigated under a DC electric field. As shown in Figure 16a, the SiO2/CdS/wax composite spheres were dispersed uniformly in the capsules under no electric field. When the field was applied at E ) 30 V/mm, the composite spheres migrated to the electronegative side of the microcapsules (Figure 16b), by electro-osmotic and electrophoretic forces.26 This indicated that the spheres had a

Figure 16. Microcapsules under the electric field: (a) E ) 0; (b) E ) 30 V/mm; (c) E ) -30 V/mm.

positive charge. When the direction of the field was reversed, the spheres were pulled back to the opposite side of the capsules (Figure 16c), with a response time of approximately 200 ms. The switching behavior was not as symmetrical, probably due to a deformation of the electric field around the microcapsules and the interaction of particles in the adjacent capsules. If the applied voltage is removed, then the yellow spheres will remain at the position to which they have been moved. Actually, this stationary state may be kept for hours until the composite spheres are once more dispersed in suspension by Brownian motion. This means that the microencapsulated electronic ink has the property of bistability. Conclusions Yellow SiO2/CdS/wax (shell/middle/core) composite spheres, which can be used as electrophoretic particles, have been prepared by electrostatic attraction assembly of CdS in emulsion, followed by the coating of the surface with SiO2 in the same solution via hydrolysis of Na2SiO3. There was no obvious interface between SiO2 shell and CdS shell due to CdS crystals


dispersing into SiO2 shell. Compared with previous electrophoretic particles, our composite particles have four advantages: (a) low density like organic pigment, (b) strong durability in environments like inorganic pigment, (c) good round shape, and (d) good optical and electrophoretic property. This method of combining emulsion and electrostatic self-assembly is more effective and easier for operation, and it also can be extended to produce other inorganic/organic composite particles or hollow particles. Acknowledgment This work is supported by the Natural Science Foundation of Tianjin (No. 07JCZDJC00300) and the National High Technology Research and Development Program of China (863 Program) (Grant No.2004AA302010). Literature Cited (1) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. An Electrophoretic Ink for All-printed Reflective Electronic Displays. Nature 1998, 394, 253. (2) Sheridon, N. K.; Richley, E. A.; Mikkelsen, J. C.; Tsuda, D.; Crowley, J. M.; Oraha, K. A.; Howard, M. E.; Rodkin, M. A.; Swidler, R.; Sprague, R. The Gyricon Rotating Ball Display. J. SID 1999, 7, 141. (3) Davis, D.; Khan, A.; Jones, C.; Huang, X. Y.; Doane, J. W. Multiple Color High Resolution Reflective Cholesteric Liquid Crystal Displays. J. SID 1999, 7, 43. (4) Zang, H. M.; Liang, R. C. Microcup Electronic Paper by Roll-toRoll Manufacturing Processes. Spectrum 2003, 16, 16. (5) Robert, A. H.; Feenstra, B. J. Video-speed Electronic Paper Based on Electrowetting. Nature 2003, 425, 383. (6) Guo, H. L.; Zhao, X. P. Preparation of a kind of Red Encapsulated Electrophoretic Ink. Opticle Materials 2004, 26, 297. (7) Yu, D. G.; An, J. H.; Bae, J. Y.; Ahn, S. D.; Kang, S. Y.; Suh, K. S. Negatively Charged Ultrafine Black Particles of P(MMA-co-EGDMA) by Dispersion Polymerization for Electrophoretic Displays. Macromolecules 2005, 38, 7485. (8) Kim, M. K.; Kim, C. A.; Ahn, S. D.; Kang, S. R.; Suh, K. S. Density Compatibility of Encapsulation of White Inorganic TiO2 Particles Using Dispersion Polymerization Technique for Electrophoretic Display. Synth. Met. 2004, 146, 197. (9) Kim, K. D.; Kim, H. T. Comparison of the Effect of Reaction Parameters on Particle Size in the Formation of SiO2, TiO2, and ZrO2 Nanoparticles. Mater. Lett. 2003, 57, 3211. (10) Kolen’ko, Y. V.; Maximov, V. D.; Burukhin, A. A.; Muhanov, V. A.; Churagulov, B. R. Synthesis of ZrO2 and TiO2 Nanocrystalline Powders by Hydrothermal Process. Mater. Sci. Eng., C 2003, 23, 103.

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ReceiVed for reView March 13, 2008 ReVised manuscript receiVed October 30, 2008 Accepted October 31, 2008 IE800416W

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